Planar waveguide grating devices have been widely used for wavelength multiplexing/demultiplexing, routing and optical add-drop applications in dense wavelength-division multiplexing (DWDM) networks. Commonly, this is accomplished with either an arrayed waveguide grating (AWG), or an etched reflecting or transmissive diffraction gratings shown in FIGS. 1a and 1b. When the device performs a demultiplexing function, multiple signal channels of different wavelengths launched into an input waveguide of the device are separated and each signal channel is directed to a predetermined output waveguides of a plurality of output waveguides. A typical spectral response of such a device is shown in FIG. 2.
One of the most desired features of such devices is a spectral response having wide and flat response characteristics within a passband of each signal channel. This feature allows high modulation frequency or data rate of the incoming signal. Further, a spectral response graph having a flat wide portion throughout the passband is indicative of a device with a large tolerance to wavelength drift of an input signal received at the input waveguide and tolerant to passband wavelength drift of the device resulting from, for example, temperature variation. Also, a spectral response as indicated above reduces the effect of polarization dispersion resulted from the planar waveguide geometry. Moreover, a device having such a flat and wide passband is particularly important in WDM networks where multiple filters are cascaded and the cumulative passband is much narrower than that of a single stage filter.
It is also highly desirable that the transmission coefficient drops sharply at the edges of the passband within the spectral response so that adjacent channels can be closely spaced without causing unacceptable crosstalk. Evidently, a sharper change in transmission coefficient also results in signals within the passband being passed with approximately equivalent attenuation, thereby, rendering the entire passband similar in response.
In a planar waveguide demultiplexing device such as the one shown in FIG. 1, the shape of the spectral response is determined by a convolution of the amplitude distribution at the output focal plane, an image of the input waveguide mode profile formed by the grating, with the mode profile of the output waveguide. The channel spectral response is approximately Gaussian shaped when single-mode waveguides are used for both input waveguide and output waveguide in a conventional device. The passband is narrow, the passband top is not flat and the transition is slow. Typically, a standard 50 GHz spacing demultiplexer has a 1 dB-passband of about 8.about.12 GHz.
Many improved designs have been proposed to flatten and widen the passband spectral response. However, they all have limitations and drawbacks.
In an article entitled "Phased array wavelength demultiplexer with flattened wavelength response" by M. R. Amersfoort et al., Electron. Lett. 30, pp. 300-301 (1994), multimode output waveguides are used to flatten the spectral response within the passband in an AWG demultiplexer. The same method is used in etched grating based demultiplexers in a paper entitled "Monolithic integrated wavelength demultiplexer based on a waveguide Rowland circle grating in InGaAsP/InP" by J.-J. He et al, IEEE J. Lightwave Tech. 16, pp. 631-638 (1998). This method can only be used in a receiver device where the output signals of the demultiplexer are immediately converted to electronic signals by photodetectors. It cannot be used if the output signals are to be coupled into optical fibers or single-mode waveguides as in the case of wavelength routing and optical add-drop devices.
In U.S. Pat. No. 5,412,744 entitled "Frequency routing device having wide and substantially flat passband" by C. Dragone (issued May 1995), two output waveguides are combined using an optical coupler. Since each channel takes the space of two waveguides at the output plane, it limits the total number of ports that can be provided. Also, the coupler introduces a loss of at least 3 dB. In U.S. Pat. No. 5,706,377 entitled "Wavelength routing device having wide and flat passbands" by Y. P. Li (issued January 1998), the passband was further widened at the expense of further increased loss by using Y-branch couplers/splitters in both the input and output sides.
Several patents and publications exist which widen the passband and flatten the spectral response within the passband by broadening the input source. In an article entitled "Flat spectral response arrayed waveguide grating, multiplexer with parabolic waveguide horns" by K. Okamoto and A. Sugita, Electron. Lett. 32, pp. 1661-1662 (1996), a parabolic tapered waveguide horn is used at the input waveguide. A double-peaked intensity distribution is formed at the input plane. This double-peaked distribution is imaged onto the output plane by a grating and result in a widened and flattened passband spectral response. This parabolic waveguide horn is replaced by a multimode interference coupler (MMI) in U.S. Pat. No. 5,629,992 entitled "Passband flattening of integrated optical filters" by M. Amersfoort and J. B. D. Soole (issued May 1997), and in a paper entitled "Use of multimode interference couplers to broaden the passband of wavelength-dispersive integrated WDM filters" by J. B. D. Soole et al., IEEE Photon. Tech. Lett. 8, pp. 1340-1342 (1996). In U.S. Pat. No. 6,049,644 entitled "Optical routing device having a substantially flat passband" by C. Dragone (issued April 2000), a wide input waveguide with a longitudinal slot in the middle is used to produce a double-peaked intensity distribution. In all these methods, the end width of the input waveguide is much larger than the ordinary single mode waveguides such as those used for output waveguides. This limits the number of input ports/waveguides and thus is not suitable for N.times.N routing devices. Moreover, those input waveguide end structures are very sensitive to fabrication errors.
Cascading two grating devices may also result in a flattened passband spectral response, as disclosed in a paper entitled "An original low-loss and pass-band flattened SiO2 on Si planar wavelength demultiplexer" by G. H. B. Thompson et al., OFC Technical Digest, TuN1 (1998) and in U.S. Pat. No. 5926587 entitled "Optical passband filters" by J. C. Chen and C. Dragone (issued July 1999). However, this method increases the device size and transmission losses.
In the article "Arrayed waveguide grating multiplexer with flat spectral response" by Okamoto and H. Yamada, Optics Lett. 20, pp. 43-45 (1995), the complex amplitude distribution at the grating plane at the output star coupler is adjusted according to a cardinal sine (sinc) function. The intensity distribution at the output plane has thus the form of a rectangular function according to the Fourier transform principle. While the phase adjustment (or negative sign) required by the sinc distribution at certain array arms can be easily realized by adjusting the waveguide lengths, the amplitude adjustment is much more difficult. It is realized by varying the tapered entrance widths of the arrayed waveguides. A similar method is disclosed in U.S. Pat. No. 5,467,418 entitled "Frequency routing device having a spatially filtered optical grating for providing an increased passband width" by C. Dragone (issued November 1995), in which the amplitude distribution is realized by introducing losses in the arrayed waveguides using lateral displacements between two waveguide segments. A drawback of these methods is that the amplitude adjustment is difficult to control due to fabrication errors and it may increase losses, phase errors and crosstalk of the device significantly. Also, since a large portion of the grating has a negative phase and contributes destructively to the output, the resulting peak transmission intensity is reduced significantly. Furthermore, these methods only improve the amplitude distribution at the output focal plane, i.e. the image of the input source formed by the grating. The resulting shape of the spectral response is quite different from this spacial amplitude distribution because of the effect of convolution with the mode profile of the output waveguide. The improvement in the passband transition is thus very limited.
In an article entitled "Multigrating method for flattened spectral response wavelength multi/demultiplexer" by A. Rigny et al., Electronics. Letters 33, pp. 1701-1702 (1997), and in U.S. Pat. No. 5,978,532 entitled "Spectrographic multiplexer component having an array of waveguides" by the same authors, two arrays of waveguides with different path length differences are interleaved to flatten passband spectral response. The spectral response is effectively the sum of two Gaussian functions peaked at two slightly different wavelengths. The technique has the advantage of simplicity in terms of both design and fabrication. However, the simple method is obviously not the optimum solution and it does not improve the sharpness of the transition within the spectral response to and from the passband.
Passband flattening, flattening of the spectral response within the passband, can also be achieved by multiple-focal-point design in which the shape of star couplers in AWG devices are modified, as reported in a paper entitled "Passband flattening of phasar WDM using input and output star couplers designed with two focal points", by Boerk et al, OFC Tech Dig., pp. 302-303 (1997) and in another paper entitled "Flat channel-passband wavelength multiplexing and demultiplexing devices by multiple-Roland-circle design" by Y. P. Ho et al., IEEE Photonics Tech. Lett. 9, pp. 342-344 (1999). Similar to the multigrating design, the multi-focal-point design does not improve the steepness of transitions within the spectral response graph to and from the passband.
In an article entitled "Nonlinear phase apodisation techniques for arrayed-waveguide grating passband control" by F. Farjady et al., IEE Colloquium on Multiwavelength Optical Networks: Devices, Systems and Network Implementations, Ref. No. 1998/257, pp. 4/1-4/4 (1998), a sub-parabolic phase term is introduced in the arrayed waveguides. While the passband spectral response is broadened with this method, the width of the transition to and from the passband within the spectral response graph is increased significantly and the induced loss is excessive, result in a significant degradation in overall performance.